With recent remarkable progress of fine working technique, nano-technology is attracting attention in various fields. For example, high-function materials having nano-scale structure such as quantum wires and quantum dots are investigated actively.
Conventionally, nano-structures having nano-scale construction have been formed by utilizing artificial nano-structure technique of the semiconductor processes such as photolithography, electron ray exposure, and X-ray exposure. However, these conventional semiconductor techniques are considered to be not suitable for easy formation of very fine structure of several tens of nanometers or less in a large area in view of production yields and through put. On the other hand, a natural formation technique, namely a technique utilizing self-organization phenomenon of a material, is considered to be useful as the technique for forming a finer structure simply in a larger area in comparison with the conventional semiconductor process.
The process for producing the nano-structure by utilizing the self-organization phenomenon is exemplified by anodization of aluminum and anodization of silicon.
In anodization of aluminum, a porous anodization film of aluminum can be obtained by anodization of an aluminum substrate in a solution of an acidic electrolyte such as sulfuric acid, oxalic acid, and phosphoric acid (R. C. Furneaux, W. R. Rigby, & A. P. Davidson, Nature, vol. 337, p. 147, 1989). This porous anode-oxidation film is characterized by the specific geometrical construction thereof in which very fine cylindrical pores of several to several hundreds of nanometers in diameter are partitioned by alumina partition wall at intervals of several tens to several hundreds of nanometers and are arranged in a parallel direction. The obtained cylindrical pores have a high aspect ratio and have excellent uniformity in depth and sectional diameters. The construction of the porous anodization film can be controlled to some extent by controlling the anodization conditions. For example it is known that the intervals between fine pores can be controlled by anodization voltage, the pore depth can be controlled by anodization time, the pore diameter can be controlled by etching treatment of partitioning alumina with phosphoric acid.
By anodization, porous silicon is formed by employing a p-type silicon substrate as the anode and applying a voltage in an aqueous hydrofluoric acid solution (D. R. Turner, J. Electrochem. Soc., 105, 402, 1985). This porous silicon has numerous fine pores of 1 nm to several tens of nanometers in diameter: the diameter and shape of the pores can be varied by controlling the anodization conditions.
The fine pores filled with a metal or a semiconductor are promising in various applications such as magnetic recording medium, magnetic sensors, EL light emitting elements, electrochromic elements, optical elements, solar cells, and gas sensors.
Many studies have been made on the nano-structure formed by self-organization phenomenon as mentioned above. However, in anode-oxidized alumina of pore intervals of less than 10 nm, the formed pores are not perpendicular to the substrate, rendering the partition wall defective, and making it extremely difficult to partition the respective pores to be independent by the partition wall. On the other hand, in anodization of silicon, the kind of the substrate is limited, and the pores tend to become branched.
The present invention intends to provide a process for producing the nano-structure having pore diameters and pore intervals controllable to some extent without changing the composition ratio of the aluminum-silicon-germanium mixed film, the aluminum-silicon mixed film, or the aluminum-germanium mixed film.